1. Field of the Invention
The present invention relates to manufacturing of airfoil structures suited for gas turbine engines and, more particularly, to a new cast hollow airfoil structure with openings which do not require plugging.
2. Description of the Prior Art
Gas turbine engine airfoils, such as gas turbine blades and vanes, may be provided with an internal cavity defining cooling passageways through which cooling air can be circulated. By cooling these airfoils, they can be used in an engine environment which is hotter than the melting point of the airfoil metal.
Typically, the internal passages are created by casting with a solid, ceramic core which is later removed by well known techniques, such as dissolving techniques.
The core forms the inner surface and tip cavity of the hollow airfoil, while a mold shell forms the outer surface of the airfoil. During the casting process, molten metal fills the space between the core and the shell mold. After this molten metal solidifies, the mold shell and the core are removed, leaving a hollow metal structure.
The region of the core which later forms the tip cavity is connected to the main body of the core by tip supports. These tip supports later form the tip openings in the metal airfoil.
The casting core must be accurately positioned and supported with the mold shell in order to ensure dimensional precision of the cast product. The core is held within the shell mold by the regions of the core which later form the passage through the fixing, the trailing edge exit slots, and the tip cavity. The core is rigidly held at these extremities. During the casting process in which molten metal is poured around the core, a significant force is exerted on the core which may break the tip supports.
In order to minimize the manufacturing cost of each airfoil, the tip supports should be sufficiently large to avoid breakage during the casting process. It is also necessary to minimize the quantity of coolant air which exits the airfoil tip openings, in order to preserve the overall gas turbine engine performance.
It is possible to cast large tip openings, then plug these openings using a welding, brazing or similar process, however there would be an extra cost associated with this additional process.
Accordingly, there is a need for a new internal structure for gas turbine engine airfoils which allows for improved strength of the core during the casting process, without requiring plugging of tip openings.